In recent years, in applications of automobile use (for example, automobile pillars, door impact beams, bumper beams, etc.), steel sheets achieving both high strength and high shapeability have become desired. As one steel for filling this need, there is TRIP (transformation induced plasticity) steel utilizing the martensite transformation of retained austenite. Using this TRIP steel, it has become possible to produce such auto parts from high strength steel sheet having a 1000 MPa class or so strength and superior in shapeability. However, securing shapeability by further higher strength, for example, 1500 MPa or higher super high strength steel is difficult at the present.
In view of this situation, the technique gathering the most attention recently as a technique achieving both high strength and high shapeability is hot-stamping (also called hot pressing, die quenching, press quenching, etc.) This hot-stamping heats a steel sheet until it reaches a 800° C. or higher austenite region, then hot shapes it to thereby improve the shapeability of high strength steel sheet and cools it after shaping to quench it and obtain the desired material properties.
Hot-stamping is promising as a method for shaping super high strength members, but usually the steel sheet is heated in the air, so oxide (scale) forms on the surface of the steel sheet. Fox this reason, a step of removing the scale is required, but countermeasures are required from the viewpoints of the descaling ability, the environmental load, etc.
As art for improving on this, the art of using Al (aluminum) plated steel sheet as the steel sheet for hot-stamping so as to suppress formation of scale at the time of heating has been proposed (for example, see PTLs 1 to 3). Further, at the time of heating at hot-stamping, the Al plating melts and runs (plating part melts and becomes fluid), so the art of retaining the sheet at a temperature below the melting point of Al (aluminum) so as to avoid running has also been disclosed (see PTL 4).